The present invention relates to a method of combusting tallow, and particularly tallow that may be contaminated with the prion protein considered to be responsible for spreading the Bovine Spongiform Encephalopathy (BSE) infection in cattle, in a standard high-speed compression ignition engine. The invention also extends to combustion of other animal/vegetable-based fats and oils which are at risk of contamination by toxic chemicals which may potentially enter the food chain. The heat and power produced by the engine is preferably then used to generate electricity.
Tallow is a clarified animal fat manufactured by rendering the remains of animals. As a result of the cattle-culling program introduced in the United Kingdom, to eradicate BSE from the national cattle herd, the UK stock of segregated tallow from the cull has grown to significant proportions. There may also be a link between BSE and a new variant of Creutzfeldt-Jacob Disease (CJD) in humans. The European Union has therefore stipulated that the UK stock of tallow, and any other potentially contaminated tallow stored in Europe, has to be disposed of by incineration to avoid potential risk of human or bovine infection.
The prion protein that may be responsible for the spread of BSE infection is heat resistant and is believed to be capable of withstanding a temperature of 800xc2x0 C. For safety reasons the tallow therefore has to be destroyed by an effective means of high temperature incineration.
Also to prevent the pollution of waterways, which can occur if used cooking oil is poured down drains, caterers and consumers are being encouraged to save waste cooking oil for collection and subsequent reprocessing into either animal feed or detergent products. However, there is always a danger that used cooking oil taken from public collection facilities may have been inadvertently contaminated by other toxic chemicals, especially chemical components found in waste mineral oils, such as transformer or lubricating oils. These types of waste mineral oil can be heavily polluted with toxic, carcinogenic, polycyclic organic compounds, in particular dioxins, furans and polychlorinated biphenyls (PCBs). Cross contamination caused a food scare in Belgium in early 1999, when it is believed that waste transformer oil containing dioxins and PCBs became mixed with used cooking oil in a public recycling container. Dioxins and PCBs are thermally stable chemicals that accumulate in fat. After the recycled oil was processed into animal feed the contaminants remained in the feed and were consequently passed into the human food chain.
Dioxins, furans, PCBs and similar polycyclic organic compounds can only be effectively destroyed by high temperature incineration.
A compression ignition engine works by injecting fuel, under high pressure, into air that has been compressed by a piston travelling up a cylinder. The fuel and air mixture is further compressed until it becomes hot enough to ignite the fuel. This results in a rapid increase in temperature and pressure inside the cylinder and the piston is forced back down the cylinder. Peak cylinder pressures inside the combustion chamber can be over 140 bar and the mean combustion temperature can reach over 2000xc2x0 C.
Combusting tallow or potentially contaminated cooking oils under these high temperature conditions would be an effective way of ensuring that the tallow or oil was completely incinerated, and any contaminants contained therein would be effectively destroyed. A further benefit is that the fuel would be injected into the engine in small, discrete doses and in the event of machine breakdown or maintenance only a small amount of potentially contaminated fuel would need to be isolated from the fuel inlet pipe.
However, high-speed compression ignition engines tend to be very fuel specific and only operate efficiently on the petrochemical-based fuels that have been designed for this particular type of engine.
An engine manufacturer will normally supply a specification rating for the engine, including a recommended power output and optimum speed setting for continuous operation of the engine. This is based on a specific type of fuel i.e. diesel oil for compression ignition engine. The engine is not designed to run on other types of fuel. Often a maximum power output (at the optimum speed) is also specified, and beyond this level it is expected that inefficient combustion would occur and undesirable black smoke would be produced. Tallow, being an animal fat, has a different chemical structure to petrochemical oils and its properties, such as calorific value, viscosity, cetane number, flash point and specific gravity, are unlike mineral oils. Consequently, tallow and other animal/vegetable based oils or fats do not burn efficiently in standard high-speed conventional diesel engines and carbonaceous deposits are quickly formed in the combustion chamber, on the piston and around the valves and the fuel injector. The presence of high levels of pollutants in the exhaust gas, such as carbon monoxide, unburned hydrocarbons and particulates, is further confirmation of incomplete and inefficient combustion.
The present invention seeks to provide a method by which tallow and other animal/vegetable-based fats and oil at risk of contamination can be successfully burnt in a compression ignition engine. From a first broad aspect, therefore, the present invention provides a method of combusting tallow in a compression ignition engine wherein the combustion atmosphere is enhanced with oxygen.
In addition or alternatively to tallow, mixtures of animal- and vegetable-based oils and fats, particularly cooking oils, may be burnt in a compression ignition engine in the method of the invention. Whilst it is contemplated that such mixtures would primarily consist of animal fats and oils (e.g. greater than 50%, 60% or 70% animal-based fats/oils) it will be appreciated that where desirable, much smaller proportions of animal fats/oils may be used in combination with other types of oil such as vegetable oils, as long as the combustion properties of the mixture are suitable for oxygen-enriched combustion.
From a second aspect therefore, the invention provides a method of combusting waste cooking oil in a compression ignition engine wherein an enriched oxygen atmosphere is provided in the combustion chamber of the engine.
Enhancement of the oxygen levels in the combustion atmosphere has been found, unexpectedly to allow the successful combustion of tallow and potentially contaminated oils, thereby affording a mechanism by which it may be disposed of effectively. As tallow has a lower calorific value than diesel fuel oil, for a given power output more tallow is required than diesel oil. However, fuel economy is of less concern than the safe destruction of any potential contaminants in the tallow.
Effective destruction of any prion protein that may be present in the tallow or potential contaminants in other oils and fats is dependent on both a high mean temperature inside the combustion chamber and the length of time the combustion process is held at that high temperature.
In a preferred embodiment, a high mean combustion temperature is achieved by injecting more fuel into the engine to produce a higher than normal power output, whilst running the engine at its optimum speed. The increased oxygen concentration ensures that the extra fuel can be effectively combusted, thus generating higher than normal thermal energy inside the combustion chamber. In this regard, as the invention is primarily intended for the generation of electrical power, the engine will be set to operate at its best continuous speed.
Oxygen enhancement also leads to earlier ignition of the fuel, so that the fuel has longer to bum. Accordingly, in the preferred embodiment no adjustment is made to the normal mechanical timing of the engine to compensate for this earlier fuel ignition. This will ensure a longer and more complete fuel burn, and this is particularly beneficial to effectively combust the extra fuel required to produce higher power outputs. In this context xe2x80x98normalxe2x80x99 is the conditions or engine settings that would customarily be used to run standard diesel oil.
The continuous power output could even be beyond the maximum recommended by the engine manufacturer for regular diesel fuel. Running the engine at higher than normal continuous power raises the mean temperature in the combustion chamber and also increases the exhaust gas temperature.
This combination of increased heat of combustion and higher exhaust temperature ensures that prion protein or any other potential biological contaminants that may be in the tallow or contaminants in the oil will be effectively destroyed. The increased heat and power produced by combusting tallow in this manner can be used to efficiently generate electricity.
From a second aspect therefore, there is provided a method of generating electrical power comprising combusting tallow or potentially contaminated oil in a compression ignition engine having an enhanced oxygen combustion atmosphere, the engine being operated at a higher continuous power output than the normal recommended output of the engine so as to increase the combustion temperature, and connecting said engine to an electrical power generator.
An advantage of using tallow for electrical energy generation is that it is a renewable, sustainable, non-fossil fuel. Tallow has a limited carbon chain chemical structure and contains very low levels of sulphur, chlorine and heavy metals. It therefore burns cleanly in an enriched oxygen atmosphere without producing significant quantities of some of the pollutants associated with fossil fuels, such as sulphur dioxide and chlorine acid gases.
In addition, when tallow is burned, the carbon dioxide produced is not a net contributor towards the greenhouse gas effect. The carbon dioxide released during combustion is merely replacing the carbon dioxide originally sequestered by the plants that were eaten by the animals during the natural growing cycle.
In a typical system in accordance with the invention, one or more diesel engines are connected to an oxygen enriched air supply system and to a supply of tallow that may be either in a pure or a contaminated state or another potentially contaminated oil supply. The output shaft of the engines would be coupled mechanically to one or more electrical power generating devices. Because of the improved thermal efficiency and increased energy density, which can be attained with oxygen enrichment of the combustion atmosphere, the exhaust gas stream is hotter than is usual in a compression ignition engine. Preferably, therefore, this hot exhaust gas is used produce steam to power a further electrical generator system. Furthermore, any excess steam is preferably used for local heating.
Before entering a flue, undesirable gaseous products of high temperature combustion can be abated from the exhaust gas stream. For example, oxides of nitrogen (NOx) can be removed from the exhaust gas by means of catalytic reduction with ammonia. In this way, the system can be used to generate electricity cleanly whilst at the same time safely destroying any potential contaminants that may be present in the fuel.
In order to achieve combustion of tallow or potentially contaminated oils in the engine, the level of oxygen enrichment required may be as little as 1% above normal (i.e. 22% oxygen). However in order to achieve effective combustion having a higher combustion temperature and a longer combustion time, the level of oxygen enrichment supplied to the engine will preferably be at least 2% (i.e. 23% oxygen), more preferably between 3 and 6% above normal (i.e. between 24% and 27% oxygen) and still more preferably between 4 and 5% above normal (i.e. between 25% and 26% oxygen). At the preferred relatively low level of increased oxygen concentration, (i.e. between 25% and 26% oxygen), the oxygen rich air is safe to handle and would not cause oxidation damage to the engine components. It is also reasonably economic to supply he oxygen required for this low level of oxygen enrichment.
The enriched oxygen air can be supplied by a number of commercially available means, including gas separation membranes, pressure swing adsorption, vacuum swing adsorption and cryogenic systems.
A sensor that monitors the temperature and composition of the exhaust gas stream may, in part, control the combustion process That sensor may, for example, be linked to a valve in the air supply to the engine and said valve can adjust the oxygen concentration in the intake air to ensure the desired optimum combustion conditions. Other aspects of the engine operation, such as speed (engine revolutions), power output, fuel consumption and engine temperatures, will be monitored and controlled by the engine management system that is usually supplied by the engine manufacturer. However, it is generally desirable to determine the desired or optimum carbon monoxide (CO) level in the exhaust gas stream as this is a good indicator of the efficiency of combustion, and to control and maintain the desired CO level by adjusting the oxygen inlet concentration accordingly. The overall oxygen concentration will remain steady but minor adjustments may be made within say a limited range of e.g. xc2x10.5%. in order to maintain the outlet CO levels approximately constant.
Although all contaminants should have been destroyed in the combustion chamber, it is preferable that after leaving the engine, the exhaust gases are held for a period of time at an elevated temperature to provide further sterilisation of the exhaust gas stream. Preferably the temperature is up to at least 500xc2x0 C. and may be up to 800xc2x0 C. or even higher. Most preferably however the temperature will be between 500 and 800xc2x0 C. This may be achieved by suitable thermal insulation of the exhaust.